The CDC released a report on a Listeria monocytogenes outbreak that was documented in 2015, mainly based on a single ice cream factory, distributing the infection to different states and getting to people in the form of milkshakes.

What’s more is that L. monocytogenes does not reproduce in frozen products, but rather it survives for longer periods of time. Because ice cream has a long shelf life, infection rates were able to pass unsupervised during the bacterial outbreak.

The CDC released a report on a Listeria monocytogenes outbreak that was distributed to different states and getting to people in the form of milkshakes. Photo credit: Delish

Analyzing this episode allowed the CDC to understand exposure levels of ice cream products to L. monocytogenes, including the infection risk scenarios.

Listeria in ice cream lives longer

A total of 10 patients were diagnosed with listeriosis related to the outbreak. There were 3 cases in Texas, 5 in Kansas, one in Arizona, and one on Oklahoma. Physicians analyzed the bacteria of the Kansas patients and discovered that four of the infections came from the same factory, while the remaining patient had ingested products from the same company, but from a different factory.

The study was based on the four Kansas patients that were hospitalized from eating infected ice cream. All of them were older than 67 and younger than 84 years of age. All of them had underlying medical conditions whose side effects included a deteriorated immunological response to bacterial infection altogether.

One of the patients had two milkshakes, a second patient had three milkshakes. It is known that even if a single cell of Listeria survives on a product, it can infect a person who ingests said product. The infected factory was determined to have produced at least eight types of products, but the CDC did not collect samples from all the goods, and they also did “not know when contamination of the production line at factory 1 began.” Infected products could have arrived at hospitals, being eaten by patients deemed highly susceptible to listeriosis, including pregnant women and older adults.

The CDC reports that it was in 2010 that the first case of listeria linked to the ice cream brand was announced. Then in 2014 the first case related to a particular factory surfaced, other patients appearing that same year and in 2015, all ingesting products from the same factory.

Different exposure scenarios were proposed, based on the fact that all of the samples collected from the ice cream labeled simply as “product 1” were infected. This product would have arrived at schools, grocery stores, restaurants, and hospitals.

“In the high-exposure scenario, we assumed contamination began 2.5 years before the outbreak was recognized, that is, midway between 2010 and the date the outbreak was recognized,” reads the report issued by the CDC.

Although the epidemic was not severe, the event served for the CDC to provide further insight into the implications of listeria infection in ice cream products, seeing that they remain cold up until the moment of ingestion, allowing for the conservation of the bacteria.

“This outbreak investigation provided unique data to characterize the dose-response relationship between L. monocytogenes in general and susceptible populations. Because ice cream preserves the viability of L. monocytogenes but does not support its growth, levels of contamination were likely to have been accurately measured and have remained relatively constant over the extended shelf lives of the products,” reads the report.